1. Field of the Invention
This invention relates to a process for converting pitch into fibers. More particularly the invention concerns a process for flash-spinning pitch into plexifilamentary fibers and the fibers produced thereby. The fibers, particularly those formed from mesophase-forming pitch, are suitable as precursors for "carbon" or "graphite" reinforcing fibers.
2. Description of the Prior Art
Processes for preparing carbon or graphite fibers of very high Young's modulus of elasticity and very high tensile strength are disclosed, for example, by Singer, U.S. Pat. No. 4,005,183, "High Modulus, High Strength Carbon Fibers Produced from Mesophase Pitch", and by Diefendorf et al, U.S. Pat. No. 4,208,267, "Forming Optically Anisotropic Pitches", which disclosures are hereby incorporated by reference. In these known processes the anisotropic or mesophase content of the pitch is increased to a concentration in the range of 40 to well over 90% by a first step in which a coal-tar or petroleum pitch is heat soaked or solvent extracted. Singer discloses heating the starting pitch in an inert atmosphere at temperatures in the range of 350.degree. to 450.degree. C. for a time sufficient to produce a pitch with a mesophase content in the range of 40 to 90%, the lower temperature requiring as much as a week and the higher temperatures, between 1-40 hours. Diefendorf et al discloses washing the starting pitch with solvent (e.g., benzene) and drying the benzene insoluble fraction. After the heat soaking or solvent extraction, fibers are prepared by (a) melting the thusly prepared mesophase pitch at a temperature in the range of about 340.degree. to 380.degree. C., (b) melt spinning, centrifugal spinning or blow spinning the molten pitch into fibers, (c) setting and stabilizing the fibers and (d) then graphitizing the fibers at a temperature in the range of 2,500.degree. to 3,000.degree. C. Generally, the fibers produced from molten pitch by the known processes (i.e., step b above) are very brittle and difficult to handle. Accordingly, it is an object of this invention to provide a process for preparing precursors for stabilization (i.e., step c above) that are less fragile and easier to handle than the precursor fibers of the known processes and to provide carbon fibers having a high surface area for use in absorption and filtration applications.
Techniques for flash-spinning synthetic, crystalline, organic polymers into fibers in the form of plexifilamentary strands are disclosed by Blades et al, U.S. Pat. No. 3,081,519. According to Blades et al, the crystalline polymer is dissolved in an organic solvent and then, at a temperature above the boiling point of the solvent and under at least autogenous pressure, the polymer solution is extruded through an orifice into a region of lower temperature and substantially lower pressure, whereby the solvent flash-evaporates and a plexifilamentary fibrous structure is formed and cooled. Among the many crystalline polymers disclosed as suitable for use in the process are polyethylene, polypropylene, polyhexamethylene adipamide, polycaprolactam, polyethylene terephthalate, etc. Polyhydrocarbons, such as polyethylene and polypropylene, are preferred. However, Blades et al does not disclose flash-spinning of non-crystalline materials, such as pitch, or the making of carbon or graphite fibers.